qemu: Shared or readonly disks are always safe wrt migration

No matter what cache mode is used, readonly disks are always safe wrt
migration. Shared disks are required to be readonly or to disable
host-side cache, which makes them safe as well.

src/qemu/qemu_migration.c

@@ -828,10 +828,12 @@ qemuMigrationIsSafe(virDomainDefPtr def)
     for (i = 0 ; i < def->ndisks ; i++) {
         virDomainDiskDefPtr disk = def->disks[i];
 
-        /* shared && !readonly implies cache=none */
+        /* Our code elsewhere guarantees shared disks are either readonly (in
+         * which case cache mode doesn't matter) or used with cache=none */
         if (disk->src &&
-            disk->cachemode != VIR_DOMAIN_DISK_CACHE_DISABLE &&
-            (disk->cachemode || !disk->shared || disk->readonly)) {
+            !disk->shared &&
+            !disk->readonly &&
+            disk->cachemode != VIR_DOMAIN_DISK_CACHE_DISABLE) {
             int cfs;
             if ((cfs = virStorageFileIsClusterFS(disk->src)) == 1)
                 continue;